What does "death" intuitively mean in the paper "Death and Suicide in Universal Artificial Intelligence"?
The authors do actually give an English definition in terms of the well-known agent formulation of AI:


  We intend this usage to be intuitive: death means that one sees
  no more percepts, and takes no more actions.


It would seem that this becomes possible for a reinforcement learning agent such as AIXI in a formulation that uses semi-measures of probability (which need not sum up to 1), rather than the more traditional notion.